1. Field of the Invention
The present invention relates to an FM-CW radar apparatus that uses a frequency-modulated (FM) continuous wave (CW) as a signal for transmission and, more particularly, to an FM-CW radar apparatus that can detect an overbridge, that is, a stationary object such as a bridge, a road sign, a billboard, or the like, located above the road ahead or on the roadside.
2. Description of the Related Art
A scanning radar scans a given area by turning the radar in lateral directions with very fine step angles within a predetermined time. At each step angle, a radar beam is projected from the radar-equipped vehicle toward a vehicle traveling on the road ahead, and the wave reflected from the vehicle ahead is received; the reflected wave thus received is processed to detect the presence of a vehicle ahead and to compute the distance and relative velocity of that vehicle.
With a conventional radar, however, as the beam is scanned in lateral directions as described above, it is difficult to obtain accurate height information. As a result, when an overbridge, that is, a structure such as a bridge over the road ahead, is detected, it may not be possible to clearly identify whether it is really an overbridge or a vehicle traveling ahead.
In one disclosed method (Japanese Unexamined Patent Publication No. 2002-202365), the detected object is determined as being an overbridge candidate when, of the peak frequencies generated based on the reflected radar signal, the number of frequency peaks larger than a predetermined value is not smaller than a predetermined number.
There is also disclosed a radar apparatus that radiates a transmitted wave over a prescribed range of angles in the vehicle width direction and in the height direction, and detects angles in the two directions, i.e., the vehicle width direction and the height direction (Japanese Unexamined Patent Publication Nos. 11-38141 and 11-38142).